THIS invention relates to radio frequency (RF) electronic identification systems comprising an interrogator and a plurality of transponders. The invention more particularly relates-to operational modes for transponders forming part of such a system.
A known system of the aforementioned kind includes a reader comprising a transmitter for transmitting an RF energizing signal to the transponders and a receiver for receiving a response signal from the transponders. A microprocessor in the reader identifies a particular transponder by identification code data modulated on the response signal by the transponder when in a normal operational mode. Upon receipt of the data by the reader and thus upon identification of the transponder, the reader transmits a first signal to the transponder to switch it to a catnap mode wherein it no longer is responsive and thus stops responding to the interrogation signal, even while still being energized. After energization of the transponder has been removed for a period longer than a short reset period (typically shorter than 2 to 10 seconds) the transponder reverts to the normal operational mode, wherein it will again respond with identification code data upon being energized.
The aforementioned known transponders are also adapted to be switched from the operational mode to a sleep mode upon receipt of a second signal from the reader. When so switched, the transponders internal clock is stopped and it remains in this mode wherein it is not responsive to any signal for a period as long as an internal capacitor providing the transponder with power, remains charged. It will be appreciated that this period is not of a predictable length, but is dependent on the discharge rate of the capacitor. which rate in turn is dependent on various other variables.
Other disadvantages of the mode of operation described hereinbefore and which is charactefised by the rapid reset from the catnap mode to the normal operational mode, are that scattering and reflections of the interrogation signal may lead to multiple readings of a single transponder; and that there is no method of controlling misreading of transponders, so that transponder reading or interrogation has to be performed under strictly controlled conditions. Furthermore, once the transponder has switched to the aforementioned sleep mode, it cannot be switched to the normal operational mode and it will only revert to the normal operational mode after elapse of the aforementioned period.